In underground drift construction, after preparatory work, arch supports are built in for support of the roof; these arch supports provide access and are set in concrete. Increasing numbers of lattice girders in conjunction with shotcrete are used because, in contrast to I- or U-beams, they eliminate shaded areas behind the girder and therefore a more even layer of concrete is made possible. Such lattice girders are described in EP-B-73733, for example.
Statical evidence from such a lattice girder shows the local cut magnitudes of the individual bars of the girder based on the global cut magnitudes in the total system. The distance between the stiffening elements plays a determinative role in this.
The larger the distance chosen, the more adversely the local loads affect the girder, i.e., the less favorable the transverse loads, the bending moments in the bars, and the compression and tensile loads become, which causes increased stress on the material and can finally necessitate larger lateral section dimensions, which is also uneconomical.
Even more determinative, however, is the fact that the greater the distance between the joints on the individual bar of the frame girder, the more adversely this distance affects the local buckling tendency of such bar.
To improve the load and stability capacities of a lattice girder, the stiffening elements should ideally be relatively close to each other, on the one hand, and the individual rod of the lattice girder should be supported centrally between the joints, on the other hand, so that its buckling length is halved.
An improvement was achieved in an embodiment according to GB-A-2 195 677, which proposed a connecting element in the form of a four-sided pyramid whose tip is secured to the top rod and whose lower ends are connected by cross-struts diagonally to the lower rods. It was proposed that a separate triangularly formed intermediate element be attached perpendicularly to the rods to improve resistance to buckling for such a connecting element. However, such an additional, triangular support element bound to the rods and attached vertically between the stiffening elements produces an accumulation of closely adjacent welding joints.
This is by no means desirable, however, since these closely adjacent welding joints may affect the structure of the steel (and in the worst case may even promote a dangerous martensite formation), which can cause brittleness in the rods and can thereby place the load-bearing capacity of the lattice girder in doubt. In extreme cases the welding joints can break under heavy loads, which leads to displacement of the stiffening elements.